1. Technical Field
The present invention relates to a small-sized image pickup apparatus and an imaging optical system both used in a cell phone, a personal computer, PDA, a monitoring camera and a sensor camera, and in particular, to a small-sized image pickup apparatus and an imaging optical system employing a solid-state imaging device such as an image sensor of a CCD type or an image sensor of a CMOS type.
2. Related Arts
There has been known a small-sized image pickup apparatus wherein an image pickup element (photoelectron sensing element) such as an image sensor of a CCD (charge-coupled device) type or an image sensor of a CMOS (complementary metal oxide semiconductor) type and a camera lens are integrated solidly, and it is employed in various uses such as a cell phone and PDA (personal digital assistance). The image pickup apparatus of this kind is used extensively for the cell phone especially in recent years, and progress for high pixels of image pickup element used in the image pickup apparatus is remarkable. Further, there are increasing demands for incorporating an autofocusing (focusing) mechanism in the image device.
In the autofocusing mechanism used generally, an actuator such as a stepping motor is housed, the whole of a part of an imaging lens is moved, corresponding to a distance of a subject to be photographed. In the conventional method to drive and move the lens mechanically, there is a possibility that sufficient anti-shock property which is strongly demanded for the image pickup apparatus housed in the cell phone cannot be obtained.
In contrast to this, there has been proposed an optical element wherein optical functions can be switched electrically through utilization of “electrowetting phenomenon” (in different words, “electric capillary phenomenon”) without using a driving means such as a motor. For example, the technology like this is disclosed in TOKUHYOO No. 2001-519539 (WO99/18456, U.S. Pat. No. 6,369,954) and TOKUHYOO No. 2002-540464 (WO00/58763). These documents disclose the technology to change refracting power or to hold optical axis center by changing a form of a fluid in particular, concerning an optical element utilizing the “electrowetting phenomenon” stated above.
Incidentally, optical characteristics to converge light on the image pickup element without generating various aberrations are required for the optical element used in a photographing optical system of the image pickup apparatus as stated above. However, optical characteristics required as an optical element of a photographing optical system are not disclosed in the aforementioned documents.
In other words, there is no disclosure at all for the problem in the case of applying this optical element to a photographing optical system, especially to the image pickup apparatus such as the aforesaid one wherein a camera lens is integrated solidly, and for measures for solution for the above-mentioned problem. For the foregoing, the inventors of the present invention found out the following requirements after their studies.    (1) On the refracting power-variable optical element that utilizes the “electrowetting phenomenon”, refracting actions can be changed, but it is necessary to consider aberration of a refracting surface formed under the condition of non-voltage impression.    (2) This refracting power-variable optical element has a large individual difference in manufacture, and calibration is needed.    (3) For the optical element used in the photographing optical system, what is most important is a problem of optical axis shifting which needs to be corrected dynamically.
The refracting power-variable optical element that utilizes the “electrowetting phenomenon” cannot construct an appropriate imaging lens by itself, and in many cases, it can realize the imaging lens capable of performing autofocusing only when it is combined with plural optical elements a diaphragm. In that case, there is a problem of the method how to control aberration when it is combined with other optical members. However, there is neither disclosure nor suggestion in the aforementioned documents concerning how to control aberration as a total optical system, when constructing the imaging optical system capable of conducting autofocusing by the use of the refracting power-variable optical element that utilizes the “electrowetting phenomenon”.